Conventionally, transmission and reception systems for the remote-controlled locking/unlocking of car doors, turning ON/OFF of head lights, unlocking of trunks and the like have been proposed. In this type of system, a small transmitter is carried by a car owner and used to transmit a signal which includes an identification code and a function code, and a receiver is mounted in a car for receiving and decoding the identification code portion of the transmitted signal and for performing various operations defined by the function code.
An example of a signal transmitted from the transmitter is shown in FIGS. 15 and 16.
The transmission signal 20 shown in FIG. 15 is typically generated by a micro-computer and associated circuitry located in the transmitter. The transmission signal 20 includes an initializing pulse 15 for initializing a signal reception process of the receiver, and subsequent transmitted-signal groups 17, 18, and 19. The transmitted-signal groups 17, 18, and 19 usually contain identical (or inverse) patterns of high and low ("1" and "0") signals. As shown in FIG. 15, data transmission pulses 23, 23, and 23 are provided before each of the transmitted-signal groups 17, 18, and 19 so that the subsequent transmitted signal groups 17, 18, and 19 can be identified.
FIG. 16 shows a data structure exemplifying the transmitted-signal group 17.
The transmitted-signal group 17 includes, in this example, twenty-one bits of data, wherein the first eleven data bits constitute an identification (ID) code 21 representing identification (security) information, and the residual ten data bits constitute a function code 22 representing command information. The ID code 21 is a series of data bits ("1"s and "0"s) which are arbitrarily established for each transmitter/receiver pair such that the transmitted ID code 21 is, for all practical purposes, only accepted by an associated receiver. Once the receiver has accepted a transmitted ID code 21, the receiver then executes an operation designated by the data bits of the function code 22, such as locking/unlocking of doors, unlocking of a trunk and the like. Unlike the ID code 21, the function code 22 is typically not unique for each transmitter/receiver set.
A problem with prior art transmission and reception systems is that when the transmitted-signal groups 17, 18, and 19 include the data structure shown in FIG. 16, this data arrangement can be easily analyzed and decoded by someone who obtains knowledge that the first eleven data bits constitute the ID code 21, and the residual ten data bits constitute the function code 22. For example, if an outsider (such as a thief) intercepts and analyses the transmission signal of at least two transmitters, he or she can distinguish the ID code 21 from the function code 22, and can thereby produce unauthorized devices for disarming similar security systems.
An object of the present invention is to address the conventional issues described above by providing a signal generation method for a transmission and reception system which cannot be easily deciphered, thereby making it difficult to distinguish identification information from command information within a transmitted-signal group.